1. Technical Field
This invention relates to vent caps used in automotive batteries, particularly to vent caps used to control the discharge of gases from the battery and prevent the introduction of flames, sparks, and other ignition sources into the battery.
2. Background Art and Technical Problems
Conventional lead-acid vehicle batteries comprise a number of cells disposed in a container. A series of alternating positive and negative electrodes (plates), having separators sandwiched therebetween, are disposed in each cell and immersed in aqueous sulfuric acid electrolyte. The positive plate contains lead oxide disposed on a lead alloy grid. The negative plate contains lead as the electroactive material on a lead alloy grid.
The electromotive potential of each battery cell is determined by the chemical composition of the electroactive substrates employed in the electrochemical reaction. For the lead-acid chemistry described above, the voltaic potential is approximately two volts per cell, regardless of cell volume. Vehicles manufactured by original equipment manufactuers (OEM's) typically require a twelve volt battery. Thus, each such battery advantageously comprises six cells (6 cells.times.2 volts/cell=12 volts). In addition, OEM vehicles are configured for a battery having a conventional envelope, i.e., the external dimensions of the battery match the space provided for the battery within the OEM vehicle.
In lead-acid batteries, gases are often liberated from the electrolyte during the charge and discharge reactions. Factors such as high current charge and discharge conditions, and changes in temperature, can affect the rate at which these gases are produced. To avoid excessive gas pressure buildup which can lead to electrolyte leaks, housing failure, and possibly explosion, it is desirable to vent the high pressure regions within the battery. Due to the volatile nature of these gases, it is desirable to provide a mechanism for preventing an external flame from entering the battery through the gas exhaust vent.
Presently known batteries employ exhaust conduits which provide gaseous communication between each battery cell and the outside of the battery. Various configurations have been employed in prior art batteries to minimize electrolyte leakage from the battery. Known venting configurations typically vent the gases at a location on the battery housing cover remote from the cell fill holes. For example, a labyrinth (a zig-zag shaped conduit) has been employed to maintain gaseous communication between the battery cells and the external battery environment, the gas exhaust aperture being disposed proximate a lengthwise edge of the battery. See generally Hulsebus et al. U.S. Pat. No. 4,916,034, issued Apr. 10, 1990, and Henning U.S. Pat. No. 3,879,227, issued Apr. 22, 1975.
The labyrinth prevents the liquid from leaving the battery while allowing gas to exit the battery through a porous flame arrestor disposed in the labyrinth path proximate the exhaust aperture. However, the relatively small cross-sectional area of the labyrinth tends to produce condensation of the gases, which condensation is deposited on the flame arrestor. In systems wherein the gases are exhausted downwardly through the flame arrestor, the condensation is often pumped through the labyrinth to the flame arrestor and thereafter through the flame arrestor to the outside of the battery, resulting in electrolyte loss.
An apparatus is needed which minimizes the possibility of electrolyte leakage from the battery, inhibits the introduction of flame into the battery, directs gases from the battery, and which is economical to manufacture.